A quantitative framework for risk assessment of natural gas storage wells, including the wellhead and connected piping, has been developed to assess SoCalGas’s underground gas storage sites in California. The approach has been developed to meet and exceed the risk assessment requirements of API RP 1171 (incorporated by reference in U.S. regulation 49 CFR 192.12) and the recent changes to the California Code of Regulations. Further, several of the recommendations made in the recent PHMSA study, “Risk Assessment and Treatment of Wells” (2021), have been addressed and incorporated.

The framework uses a dynamic fault tree to aggregate the barrier failure rates from over 80 potential failure mechanisms to quantify the combined probability of an accidental release to the atmosphere and the resulting hazards. The modular architecture allows operators to use threat-specific models with differing levels of sophistication. Additionally, the framework accounts for the interplay between barriers to failure in a well, the benefits of continuous monitoring, and the effect that wellhead spacing, cement quality, and well-inflow performance have on the expected consequences.

Priority to develop quantitative models is given to the threats that potentially have high probability or high consequences and to threats with effective mitigation options, such as corrosion and external interference threats. For threats with very low consequences or likelihood of occurrence, simple models with conservative assumptions are typically sufficient because the contribution to overall risk is low. This process minimizes the overall analysis complexity and allows mitigations to be tailored to the higher-risk threats.

This paper describes example assessments to illustrate how the framework can quantify the benefits of integrity management activities, such as an erosion monitoring program, continuous pressure monitoring of the annuli, and adding protections to minimize risk from vehicle collisions.

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